In-mold labeling is a process in which a heat-sealable label is applied to a blow molded article during the molding process. The resulting label, if properly applied, appears to be not a label at all, but an integral part of the bottle itself. Examples of such labeled articles include, among others, food and beverage containers, liquid detergent containers, motor oil containers, and bleach and other household cleaning product containers.
In-mold labeling eliminates a separate labeling step after an article is formed. Typically, in-mold labels are cut to size from a sheet of substrate coated with an adhesive, then stacked in a feeding magazine for the mold. One or more labels are fed into the mold and positioned against the sidewalls of the mold cavity by a mechanical or vacuum operated device. Vacuum pressure often is used to maintain the proper position of the label during this stage of the labeling process.
The mold then typically is closed, and a parison or molten plastic balloon is blown into the closed mold cavity. The parison typically is a balloon of molten thermoplastic resin, such as high density polyethylene, polypropylene, or polyethyleneterephthalate (PET). As the parison contacts the adhesive side of the label, the heat from the parison activates the heat-sealable adhesive layer which causes the label to adhere to the blown article as the pressure of the parison against the label increases. Typically, the mold sidewalls are cooled, and thus, the molten plastic article is hardened by cooling. Subsequently, the labeled article is removed from the mold.
One general problem encountered with in-mold labeling is that the substrate film used for the labels typically is not porous. Without permitting the escape of air or volatiles from between the label and the bottle, the air or volatiles may be trapped between the label and the article which results in air pockets or blisters.
Another general problem encountered with adhesive labels, whether for in-mold or other applications, is that in feeding the adhesive labels from a stack of labels from, for example, a feed magazine, the labels may tend to stick together. This problem generally is referred to as "blocking." Blocking also can be a problem in feeding labels from a roll of labels when the transfer of adhesive from the adhesive side of one layer of labels to the print side of the successive layer of labels causes the layers to stick together. Blocking is particularly problematic in commercial labeling processes where labeling is carried out at high speeds and each piece of apparatus along a production line, including the feed magazine, must be accurately synchronized and reliable. Blocking may create downtime for the line and/or an increase in defective articles.